Anharmonic Aspects in Vibrational Circular Dichroism Spectra from 900 to 9000 cm–1 for Methyloxirane and Methylthiirane

Vibrational circular dichroism (VCD) spectra and the corresponding IR spectra of the chiral isomers of methyloxirane and of methylthiirane have been reinvestigated, both experimentally and theoretically, with particular attention to accounting for anharmonic corrections, as calculated by the GVPT2 approach. De novo recorded VCD spectra in the near IR (NIR) range regarding CH-stretching overtone transitions, together with the corresponding NIR absorption spectra, were also considered and accounted for, both with the GVPT2 and with the local mode approaches. Comparison of the two methods has permitted us to better describe the nature of active “anharmonic” modes in the two molecules and the role of mechanical and electrical anharmonicity in determining the intensities of VCD and IR/NIR data. Finally, two nonstandard IR/NIR regions have been investigated: the first one about ≈2000 cm–1, involving mostly two-quanta bending mode transitions, the second one between 7000 and 7500 cm–1 involving three-quanta transitions containing CH-stretching overtones and HCC/HCH bending modes.


S1 Experimental Section
The conversion of (S )-2methyloxirane to (R)-2-methylthiirane was performed in water according to a known procedure S1,S2 with ammonium thiocyanate at room temperature (20°C).
This synthetic route was selected, among those reported in the literature, because it involves a S N 2 mechanism that prevents the racemization of the chiral center, according to the following scheme: Scheme S1: Preparation of (R)-2-methylthiirane from (S )-2-methyloxirane in H 2 O at 20°C with ammonium thiocyanate.
The synthesized methylthiirane was purified with great difficulty. It degraded, indeed, very easily and was extremely volatile. Particularly, distillation of the reaction mixture by sample warming caused very low yields. Higher yields were only achieved by separation processes based on vacuum distillation in a low impedance transfer line between cold traps, whose temperature was controlled by thermostatic baths. The difference in vapor pressure, as a function of temperature, of solvents and the synthesized compound allowed a clean extraction of the methylthiirane. In this procedure monitoring of the nature of the extracted vapor in the transferred line, accomplished by recording He Iα (21.218 eV) photoelectron spectra (PES), was crucial.
aqueous solution. The 50 mL dodecane extraction from the aqueous solution was repeated twice. The organic phase, collecting the three extracted fractions, was dried over anhydrous Na 2 SO 4 . The solution thus obtained was filtrated and placed in a 250 mL two-necked flask that was connected to the vacuum transfer line. The most volatile species (atmospheric gases) were removed in the transfer line by two freezing-pumping cycles, where liquid nitrogen was used for freezing and the higher temperature was set at few degrees above the melting point of the solution (-5°C). Methylthiirane was extracted from the solution kept at -30°C, and trapped in a 250 mL flask dipped in liquid nitrogen. and δ 2.02 (1H, dd of CH 2 ), δ 2.43 (1H, m, CH) (see Figure S1); ii 13 C-NMR (100.61 MHz, C6D6): 30.0, 26.6, 21.8 (see Figure S2).

S1.2 Enantioselective GC and GC-MS of methylthiirane
Since the enantiomeric purity affects the circular dichroism measurements, and because no data have been published on the enantiomeric purity of methylthiirane synthetized by this method, a direct analysis of the synthetized sample was performed by GC-MS (Gas Cromatography -Mass spectrometry) using a chiral stationary phase. An extensive study was needed to select the suitable stationary phase. The following cyclodextrin derivatives were tested as chiral selectors: The GC columns were coated with each chiral selector diluted at 30% in PS-086. The column providing the best results was that coated with 30% 2,3-di-O-acetyl-6-O-tert-butyldimethylsililβ-cyclodextrin in PS086 (column 3). The results of the enantioselective analysis of the racemic methylthiirane (synthetized from racemic propylene oxide (Sigma-Aldrich, 99% purity) according to Scheme S1), and the synthesized R-enantiomer, are reported in Figure S3.
A two-component fit of the GC chromatogram with asymmetric Gaussian functions gave the  The IR and VCD spectra are also reported, with appropriate scaling factors applied to fit the y-axis scale.
S-11 2800 2900 3000 3100 Wavenumbers ( Figure S7: g-ratios of the observed transitions plotted as blue dots in the fundamental CHstretching region. The IR and VCD spectra are also reported, with appropriate scaling factors applied to fit the y-axis scale. 5600 5800 6000 6200 Wavenumbers Figure S8: g-ratios of the observed transitions plotted as blue dots in the first overtone CH-stretching region. The IR and VCD spectra are also reported, with appropriate scaling factors applied to fit the y-axis scale. S-12   Figure S11: Comparison of the experimental and harmonic IR and VCD spectra (black lines) of (R)-2-methyloxirane (first and third column) and (R)-2-methylthiirane (second and fourth column) in the fundamental CH stretching region. Calculations were performed at the PW91 level of theory. The spectra were simulated by assigning Lorentzian distribution functions with 12 cm −1 of half-width at half-maximum. A scaling factor of 0.96 was applied.  Figure S12: Comparison of harmonic IR and VCD spectra of (R)-2-methyloxirane (top panels) and (R)-2-methylthiirane (bottom panels) in the mid-IR region obtained with (green lines) and without (red lines) including CCl 4 solvent effects represented by the polarizable continuum model (PCM). Calculations were performed at the PW91 level of theory. The spectra were simulated by assigning Lorentzian distribution functions with 7 cm −1 of halfwidth at half-maximum. A scaling factor of 0.98 was applied.   Figure S13: Comparison of the experimental (black lines) IR (first and second columns) and VCD (third and fourth columns) spectra of (R)-2-methylthiirane and (R)-2-methyloxirane with the corresponding anharmonic calculations in the CH regions. Calculations were performed at the PW91 level of theory. The spectra were simulated by assigning Lorentzian distribution functions with 10 cm −1 , 15 cm −1 and 40 cm −1 of half-width at half-maximum in the fundamental, first and second overtone regions, respectively. Different colors are used in the line spectra to separate transitions to fundamental (blue lines), overtones (green) and combination (purple) states.

S5 Local Mode Results
Following the procedure reported in refs. S4 and S5, ω 0 and χ were obtained performing an adequate number of z positive and negative displacements and evaluating quadratic ϕ ll (∂ 2 V /∂z 2 ), cubic ϕ lll (∂ 3 V /∂z 3 ), and quartic ϕ llll (∂ 4 V /∂z 4 ) force constants by a polynomial fitting to a Morse potential in the internal stretching coordinate z. For each CH bond, system is oriented in CH frame with the z-axis oriented along the CH bond and the y-axis  Figure S16: Comparison of the experimental IR (first and second columns) and VCD (third and fourth columns) spectra of (R)-2-methylthiirane (second and fourth columns, in orange) and (R)-2-methyloxirane (first and third columns, blue) with the corresponding localmode calculations in the CH-stretching regions. The spectra were simulated by assigning Lorentzian distribution functions with 10 cm −1 , 15 cm −1 and 40 cm −1 of half-width at halfmaximum in the fundamental, first and second overtone regions, respectively. The dipole and rotatory strengths are also reported, in cgs units. Green lines are used for CH 3 hydrogen atoms, red lines for CH 2 and blue for CH.  Figure S17: Comparison of the experimental IR (first and second columns) and VCD (third and fourth columns) spectra of (R)-2-methylthiirane (second and fourth columns, in orange) and (R)-2-methyloxirane (first and third columns, in blue) with the corresponding localmode calculations in the CH-stretching regions. The spectra were simulated by assigning Lorentzian distribution functions with 10 cm −1 , 15 cm −1 and 40 cm −1 of half-width at halfmaximum in the fundamental, first and second overtone regions, respectively. In red and deep blue lines are reported the spectra in wich only the zero-th order of equations 3 and 4 in the main text were considered, therefore removing the contribution of electrical anharmonicity.